1. Field of the Invention
The present invention relates to a thin film electrode for devices and an electroluminescence (hereinafter referred to as EL) device having said electrode, and a process for the production of said electrode and said EL device. Specifically, it relates to a thin film electrode for devices suitable for using as a cathode of an organic EL device and a process for the production thereof, and also an EL device providing an emission of light with high brightness and high efficiency and a process for the production thereof. 2. Description of Related Arts
Research into devices, paying attention to high fluorescence efficiency of an organic compound and utilizing an EL property of the organic compound, have been made for a long time. For example, Helfrish, Dresner, Williams et al. have obtained an emission of blue light using anthracene (J. Chem. Phys., 44, 2902 (1966)). Vincett, Barlow et al. have prepared a luminescence device by the vacuum vapor deposition method using a condensed polycyclic aromatic compound (Thin Solid Films, 99, 171 (1982)).
However, both devices have low brightness and low luminous efficiency.
Recently, it has been reported that an emission of blue light of 100 cd/m.sup.2 was obtained using tetraphenylbutadiene for a light emitting material (Japanese Patent Application Laid-Open No. 194393/1984). Moreover, it has been reported that an organic thin film EL device emitting a green light with a brightness of not less than 1000 cd/m.sup.2 was developed by lamination of a hole-conducting diamine compound and a fluorescent aluminum chelate complex as a light emitting material (Appl. Phys. Lett., 51, 913 (1987)).
Moreover, it is reported that an EL emission of approximately 80 cd/m.sup.2 was obtained using a distyryl benzene compound famous as a laser pigment, which has high fluorescence in the region of blue to bluish green, in a monolayer as a light emitting material (Europe Patent No. 0,319,881).
As a cathode of the above organic thin film EL device, an alkali metal, for example, Na:K alloy has been known as described in U.S. Pat. Nos. 3,173,050, 3,382,394 and the like. The device using said electrode has a maximum quantum yield of 0.01 to 0.08% (RCA Review, vol. 30, p. 322), and an extremely low work function. Besides, said device has high activity but no stability, though it has an electorn-injectable property. Therefore, said device is not suitable for practical use.
A tunnel injection cathode (U.S. Pat. No. 3,710,167, Proceedings of IEEE, p. 1863, 1970), which is made by coating the wafer comprising n-type Si with Sio.sub.2 in a thickness of 20 to 40 .ANG., or by coating Al with Al.sub.2 O.sub.3 in a thickness of 50 .ANG., has a quantum yield of 0.01 to 0.04%, using anthracene as a light emitting layer. However, though said cathodes have higher stability compared with the above cathodes using alkali metals, but they have defects that the formation of an extremely thin oxidized film is required and the reproduction is difficult.
Moreover, in U.S. Pat. No. 4,539,507, it is reported that an EL device using In as a cathode and comprising the structure of ITO/1,1-bis(4-di-p-tolylaminophenyl)cyclohexane (hole-transporting layer)/4,4'-bis(5,7-di-t-pentyl-2-benzooxazolyl)stilbene (electron-transporting layer)/In emits light with 340 cd/m.sup.2 (quantum yield: 1.2%, power conversion effficiency: 0.14%) having a peak of 520 nm at an applied voltage of 22 V and a current density of 140 mA/cm.sup.2. In said EL device, voltage to be applied can be decreased and brightness can be increased, but In as a cathode is poor in electron-injecting property and it leaves something to be improved.
Moreover, Europe Patent No. 0,278,757 discloses a cathode comprising plural metals other than alkali metals, at least one of which is a metal having a work function of not more than 4 eV, for example, a uniform, minute cathode with a good plane resistance value which is formed by vapor depositing Mg and any of Ag, In, Sn, Sb, Te and Mn on an organic compound. Concretely, a device using Mg and Ag as a cathode and comprising the structure of ITO/1,1-bis(4-di-p-tolylaminophenyl) cyclohexane/Al complex of oxine/Mg:Ag shows a light emission (green) of 0.05 mW/cm.sup.2 (approximately 50 cd/m.sup.2) having a power conversion efficiency of 0.45% at an applied voltage of 5 V.
In the cathode of an EL device, adhesion to an organic thin film, uniformity and minuteness are increased, and thus the improved stability, the decreased plane resistance and the increased luminous efficiency can be attained. However, in said cathode, for example, the volume resistivity of Mg:Ag (atomic ratio of 10:1) is 3.4.times.10.sup.-5 -6.times.10.sup.-5 .OMEGA..cm, which is 8 to 13 times as large as that of Mg bulk (polycrystal) of 4.45.times.10.sup.-6 .OMEGA..cm. This is because the uniformity and minuteness of the thin film are insufficient, which causes oxidation in the air. In some cases, adhesion to the organic layer is not good and an emission of light is not uniform. Moreover, since Ag, In or Al is used as a second metal, the temperature of the vapor deposition source is raised to as high as 800.degree. C. or higher during vapor deposition and an organic thin film as an under layer is often damaged by radiation heat.
The present inventors have carried out research to develop a thin film electrode which has the same features as the above electrode comprising both a metal having a small work function other than alkali metals and the other metals, and which further has a uniform and minute thin film having a good adhesion to an organic layer and a good plane resistance value, without damaging an organic thin film as an under layer during formation of a thin film.
As a result, it has been found that the above object can be attained by using a specific metal and an organic compound. The present invention has been accomplished on the basis of such findings.